lxc-start-ephemeral: Fix broken mount logic

This reworks the mount logic for lxc-start-ephemeral to be as follow:
 - Any real (non-bind) entry gets copied to the target fstab
 - Any bind-mount from a virtual fs gets copied to the target fstab
 - Any remaining bind-mount if confirmed to be valid gets setup as an
   overlay.

Extra bind-mounts passed through the -b option are mounted by the
pre-mount script and don't need processing by the fstab generator.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>